This invention relates to a rare-earth oxysulfide scintillator and an X-ray detector using the scintillator.
As explained in the article "Ceramic Scintillators for Advanced, Medical X-ray Detectors," American Ceramic Society Bulletin, pp. 1120-1130 (1992) (incorporated herein in its entirety by reference), X-ray computed tomography (CT) is an important and useful medical diagnostic technique capable of reconstructing cross-sectional images, e.g., of the body. An important component of X-ray CT is a scintillator.
In general, an X-ray detector measures the intensity of X-rays passing through, e.g., a patient. The detector contains scintillator elements having luminescent ions, which emit visible light proportional to the amount of X-rays absorbed in each scintillator element. Scintillators then direct the emitted light onto photodiodes for conversion of light energy into electrical energy (electrical signals). The electrical signals are generally read every 1 ms and are digitized for computer generation of cross-sectional absorption coefficients suitable for display on a cathode ray tube (CRT) screen.
Scintillators emit light with X-ray irradiation, which is combined with a photodiode to detect X-rays for X-ray CT. Short afterglow and high X-ray radiation resistance are important properties required for effective scintillators. Equally important, scintillators must be highly sensitive to X-ray irradiation.
The American Ceramic Society Bulletin article, supra, describes acceptable properties of scintillators for use in X-ray detectors. They include:
a high X-ray absorption coefficient (stopping power) of .gtoreq.20 cm.sup.-1 ; PA1 &lt;2% change in luminescent intensity after a strong X-ray dose of 500 Roentgens; PA1 a short primary decay time of .ltoreq.1 ms; and PA1 a luminescent afterglow, or fractional light output, of &lt;0.1% at 100 ms after X-ray turn-off. PA1 (1) in a glow curve, PA1 (2) in an emission spectrum,
With respect to afterglow, the present inventors, however, have discovered that the luminescent afterglow should be less than 0.01% at 100 ms after X-ray turn-off.
Japanese Laid-open patent KOKAI 58-204088 shows a Gd.sub.2 O.sub.2 S:Pr scintillator, which is a rare-earth oxysulfide ceramic. Also, U.S. Pat. Nos. 4,752,424 and 4,863,882 disclose a method for producing such a scintillator.
Japanese Laid-open patent KOKAI 56-151376 shows a method for reducing afterglow, which is to dope rare-earth oxysulfides, such as Gd.sub.2 O.sub.2 S:Pr, with Ce. Based on experiments by the present inventors, doping with Ce is effective in reducing afterglow only 1 to 10 ms after stopping X-ray irradiating. However, Ce doping is not effective in reducing afterglow 100 ms after stopping X-ray irradiating. In particular, to reduce the intensity of afterglow to less than 0.01%, the Ce content is required to be more than 20 ppm. However, such a large amount of Ce-addition causes the intensity of emission to decrease.
Further, the addition of Ce is not effective in improving radiation resistance, i.e., reducing radiation damage. Japanese Laid-open patent KOKAI 3-192187 shows a method for reducing afterglow by reducing the amount of Eu included in a scintillator. But, this method is not effective in reducing afterglow 100 ms after stopping X-ray irradiating.
Japanese Laid-open patents KOKAI 2-173088, 2-209987, 2-212586 and 3-243686 show methods for improving radiation resistance. However, the reduction of the scintillator light output after 500 roentgens irradiation can not be suppressed to less than 2% by each of these methods.